The invention relates to so-called ball valves, wherein a valve member which rotates to control fluid flow through the valve is characterized by a spherical surface which has sealed engagement to an annular seat for the closed condition of the valve.
The conventional valve member of a ball valve is a full sphere, except for a radial stem and a diametrically extending bore that is transverse to the stem direction. The valve-member or ball is actuable by limited rotation, e.g., 90 degrees, about a valve-body axis of stem support, wherein said axis extends through the center of the sphere, intersecting and normal to the axis of the diametrically extending bore.
In the open condition of the valve, the diametrically extending bore aligns with cylindrical inlet and outlet ports or passages in the valve body, and an annular seal such as an elastomeric O-ring retained by one or both of these ports or passages is in peripherally continuous seated and sealing contact with the ball, encircling the adjacent end of the diametrically extending bore of the valve member. As the valve-member is actuated in the valve-closing direction, the valve-member bore and the inlet/outlet passage become progressively misaligned while the ball rotates with continuing seat engagement, thus reducing the sectional area available for inlet-to-outlet flow. When fully rotated to the valve-closed condition, a smooth spherical surface of the ball is circumferentially sealed to its seat, in total blockage of inlet-to-outlet flow.
The actuating operation of a conventional ball valve is thus characterized by the frictional resistance of the ball-to-seat engagement. For many applications, this friction can be reduced by appropriate choice of seat material and by careful attention to ball sphericity and to the accuracy of ball-stem mounting and rotation. But for other applications, as for controlled flow of cryogenic materials such as liquified oxygen, liquified hydrogen, liquified nitrogen, or other gases whether or not in liquid state, seating materials and engagements become sources of friction, wear, and leakage, to the extent that mechanical hysteresis is an on-going operational factor, and repair and maintenance expenses are relatively great.